1. Field of the Invention
This invention relates to the field of blasting and is particularly concerned with means for transmitting an initiating signal (non-electrically) to an explosive device to remotely detonate same in accordance with a predetermined delay period.
2. Description of the Prior Art
There have been many proposals for achieving remote detonation of explosives by means of non-electric methods of detonation signal transmission. These include the so-called "shock wave conductors", which consist of plastics tubing containing a fine dusting of particulate chemicals capable of reacting to propagate a percussion wave throughout the length of the tubing, as currently available commercially under the Trade Mark "Nonel". Reactive combinations of chemicals that have to date achieved sufficiently reliable and reproducible performance for practical systems have signal propagation velocities of around 2000 m.s.sup.-1, which leads to inconveniently long lengths of tubing as delay elements. Achievement of desirable slower propagation velocities has been frustrated by the lack of suitable, reliable, precise, reactive compositions for low energy shock tubes. For an inter-hole delay of, say, 10 milliseconds at, for example, 5 meters interhole separation a propagation velocity of from around 500m sec.sup.-1 to, at most, say 1000m sec.sup.-1 would be desired for the low energy fuse to allow for short or at least manageable lengths of tubing to be used. At 20 milliseconds interhole delay the desired maximum propagation velocity would drop correspondingly to about 400 to 500 meters/second.
There have been various past approaches to reducing the overall signal transmission rate of shock tube systems--by interposing pyrotechnic delays along the tube lengths and mechanically by introducing artifacts to the tubing, such as coils, or forming constrictions in the tubing itself.
The literature contains reports of examples of various chemical compositions that give lower signal transmission rates. Thus signal velocities of around 1200 m.s.sub.-1 have been reported for reactive compositions comprising aluminium and sundry oxidants, e.g. a potassium bichromate, aluminium, sugar mixture at a charge density of 10 mg.m.sup.-1. Using a more complex pyrotechnic chemical composition made up of lead oxide, zirconium, vanadium pentoxide, silicon and amorphous boron at a charge density of 14 mg.m.sup.-1 it has been reported that a burning speed of 820 m.s.sup.-1 was achieved. In the absence of commercial products it has not been possible to assess the reliability or precision of those particular compositions in low-energy shock tube. Applicants attempts to reproduce these reported results and to achieve even lower velocities have generally been unsatisfactory due to difficulties in achieving reproducible performance. Thus in a series of experiments on apparently equivalent samples it is often found that some of the samples will fire, but at irregular speeds and others will simply not propagate the initiated signal the full length of the tubing.
In order to achieve a satisfactory delay period without use of excessive lengths of tubing, it is necessary to continue research into ways of reducing the transmission velocity still further. Thus it is an object of the present invention to provide improvements in low energy timing fuses. It is a further object of this invention to provide a shock tube delay element for use in a blasting system.